test: use synctest in some CRDB pool retry tests

[ivanauth]

Summary

Rewrites the CRDB pool retry tests to use Go's testing/synctest package for better time simulation and test reliability.

Fixes #2713

Changes

• Wrap all retry tests in synctest.Test(t, func(t *testing.T) { ... })
• Use t.Context() instead of context.Background()
• Add t.Parallel() for concurrent test execution

Benefits

• Tests no longer rely on real time delays
• More deterministic test execution
• Faster test runs with simulated time
• Better parallel execution support

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.58%. Comparing base (f0f94ee) to head (22d64e9).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2754       +/-   ##
===========================================
+ Coverage   50.01%   74.58%   +24.57%     
===========================================
  Files         417      484       +67     
  Lines       52438    58531     +6093     
===========================================
+ Hits        26224    43651    +17427     
+ Misses      23562    11810    -11752     
- Partials     2652     3070      +418     

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